The present invention relates to determining and utilizing the amount of unspent zinc-based sulfide scavenger which is present in a water-based drilling fluid for adjusting the scavenging capability of the drilling fluid to the extent desired during the drilling of a well. More particularly, the invention relates to a relatively quick and accurate procedure which can be used in field locations.
A state of the art paper entitled "Chemical Scavengers for Sulfides in Water-Based Drilling Fluids" by R. L. Garrett, R. K. Clark, L. L. Carney and C. K. Grantham, Sr. in Journal of Petroleum Technology, June 1979, page 787, discusses the chemistry of commercial scavengers for water-based drilling fluids, the parameters that affect the reliability of such materials and the problems affecting scavenger use. On page 796, the authors point out that "From this state of the art review one can see that we believe more research is needed to develop scavengers and tests for scavenger content in muds that match more closely the qualities of an ideal scavenger."
U.S. Pat. No. 3,706,532 describes a method for determining zinc concentrations in aqueous mediums. A sample medium is acidified, treated with a buffered complexing agent to complex aluminum or iron ions, treated with an organo sulfur compound to complex copper ions, then analyzed by adding an indicator the color intensity of which is calibrated with respect to known concentrations of zinc. U.S. Pat. No. 3,928,211 describes a class of zinc carbonate, basic zinc carbonate and zinc hydroxide compounds effective for sulfide scavenging. U.S. Pat. No. 4,252,655 describes the removal or inactivation of hydrogen sulfide contamination by adding at least one organic zinc chelate.